Computer System Components

The diagram is a general view of how desktop and workstation computers are organized. Different systems have different details, but in general all computers consist of components (processor, memory, controllers, video) connected together with a bus. Physically, a bus consists of many parallel wires, usually printed (in copper) on the main circuit board of the computer. Data signals, clock signals, and control signals are sent on the bus back and forth between components. A particular type of bus follows a carefully written standard that describes the signals that are carried on the wires and what the signals mean. The PCI standard (for example) describes the PCI bus used on most current PCs.

The processor continuously exectues the machine cycle, executing machine instructions one by one. Most instructions are for an arithmetical, a logical, or a control operation. A machine operation often involves access to main storage or involves an i/o controller. If so, the machine operation puts data and control signals on the bus, and (may) wait for data and control signals to return. Some machine operations take place entierly inside the processor (the bus is not involved). These operations are very fast.